MySQL : UPDATE query and allows negative - mysql

Question

The objective of this code is to deduct ringgit value and sen value in respective column and allows the value to be negative.
Assuming the price of item to purchase are RM1.50, and the users' balance are RM 0.80, RM 2.40 and RM -1.00.
The table should look like this :
Before purchase was made
Id
user
ringgit
sen
1
User A
0
80
2
User B
2
40
3
User C
-1
0
After purchase was made.
Id
user
ringgit
sen
1
User A
0
-70
2
User B
0
90
3
User C
-2
-50
I used code below to deduct user's balance. But the code only can deduct correctly if the user's balance is sufficient to purchase the item. (no negative value allowed)
UPDATE users SET ringgit = CASE
WHEN (sen - ".$senToDeduct." < 0) THEN (ringgit - ".$ringgitToDeduct." - 1)
WHEN (sen - ".$senToDeduct." >= 0) THEN (ringgit - ".$ringgitToDeduct.")
END,
sen =
CASE
WHEN (sen - ".$senToDeduct." < 0) THEN (100 - (".$senToDeduct." - sen))
WHEN (sen - ".$senToDeduct." >= 0) THEN (sen - ".$senToDeduct.") END WHERE Id = 1
I try to use code as follows :
UPDATE users SET ringgit =
CASE
WHEN (((ringgit * 100) + sen) - 150 >= 0) THEN LEFT(((ringgit * 100) + sen) - 150,char_length(((ringgit * 100) + sen) - 150)-2)
WHEN (((ringgit * 100) + sen) - 150 < 0) THEN - LEFT(((abs(ringgit) * 100) + abs(sen) - 150),char_length(((abs(ringgit) * 100) + abs(sen) - 250))-2)
END,
sen = CASE
WHEN (((ringgit * 100) + sen) - 150 >= 0) THEN RIGHT(((ringgit * 100) + sen) - 150,2)
WHEN (((ringgit * 100) + sen) - 150 < 0) THEN - RIGHT(((abs(ringgit) * 100) + abs(sen) - 150),2)
END
WHERE Id = 1
..but it does not work as the ringgit column already got affected after the query run SET ringgit =